TAW 






DEPARTMENT OF COMMERCE 

BUREAU OF STANDARDS 

S. W. STRATTON. Director 



TECHNOLOGIC PAPERS OF THE BUREAU OF STANDARDS, No. 83 

[Issued November 14, 1916] 



FAILURE OF BRASS. 2.— EFFECT OF CORROSION ON 
THE DUCTILITY AND STRENGTH OF BRASS 



By Paul D. Merica, Associate Physicist 



INTRODUCTION 

Investigation^ during the last few years has established the 
fact that brass will crack or fracture when exposed to the action 
of corroding agents, while at the same time imder tensile stress, 
even when the value of this stress is much less than the ultimate 
strength or even the yield point of the material, as determined in 
the tensile test. It is well known that surface corrosion plays an 
important part, even a necessary one, in the "season cracking" 
of brass, and Jonson ^ has carried out a number of experiments, 
in which he has subjected brass test specimens to tensile stress in 
a testing machine, the specimens being at the same time im- 
mersed in a solution of concentrated ammonium hydroxide. He 
foimd that so long as the applied stress in these tests was not 
greater than the elastic limit of the material, the specimens did 
not fail, but when a stress was applied greater than this limit — 
e. g., from 20 000 to 40 000 pounds per square inch — cracks 
would appear after a few days, the specimen would yield con- 
tinually and finally break. 

The effect of corrosion, at least of that caused by the action 
of ammoniimi hydroxide, on brass under stress, be it initial or 
due to external load, is to decrease the strength and also the 

* E. Heyn, Internal Stresses in Cold-Wrought Bars, and Some Troubles Caused Thereby, J. Inst. 
Metals. 12, 1914, p. 3; A. D. Flinn, Brass in Engineering Construction, Engineering Record, 68, 1913, p. 
527; P. D. Merica and R. W. Woodward, The Failure of Structural Brass, Trans. Amer. Inst. Metals, 
1915- 

* E. Jonson, Failures of Forgible Brass Bars, Trans, Amer. Inst. Metals, 8, 1914, p. 13s; The Fatigue 
of Copper Alloys, Proc. Amer. Soc Test. Materials, 40, 1915, p. lor. 

87248°— 22 






2 Technologic Papers of the Bureau of Standards " ' 

ductility of the material, for it is a striking feature of such brass 
failures as occur, due to combined stress and corrosion, that they 
occur with little elongation; the brass does not display its usual 
ductility. Parallel to his stress-corrosion tests, Jonson ran ten- 
sile tests on specimens of the same materials, subjecting them to 
the same loads, but not at the same time to the action of the 
ammonium hydroxide; such specimens did not fail after months 
of test. Furthermore, specimens were corroded with ammonium 
hydroxide and subsequently tested in tension, giving quite nor- 
mal results, such that it must be considered that it is the simul- 
taneous effect or action of tensile stress and corrosion (ammonium 
hydroxide) which produces failure in brass at stresses below its 
ultimate strength. 

It may here be noted that Jonson used only ammonium hydrox- 
ide in his tests; he draws the conclusion, however, that any cor- 
roding agent, even water, will in time produce the same effect of 
cracking. The admissibility of this further conclusion seems 
probable, but is not to be regarded yet as proven. 

The author wishes to advance an explanation for this rather 
striking phenomenon, of the combined action of tensile stress 
and corrosion on brass, indicating the actual manner or "mecha- 
nism" by which this effect is produced, in the hope that this 
explanation will contribute to progress in dealing with this im- 
portant technologic problem. 

The initial effect of corrosion on any metal, stressed or un- 
stressed, is to roughen the surface, such that in section it appears 
with ridges or furrows. This nonuniform action of the corroding 
agent is due to slight variations of the electrolytic (solution) po- 
tential over the original (smooth) surface; in the case of a non- 
homogeneous alloy such as an alpha-beta brass — type, 60 per 
cent copper and 40 per cent zinc — a difference of potential would 
in general exist between the alpha and the beta constituents. If 
the corroded bar were under tensile stress, the value of the fiber 
stress at the surface would, theoretically at least for a very long 
bar, be the same at all points ; this can no longer be true, however, 
of the roughened surface. 

Leon^ has shown that the stress at the bottom of a semicircular 
notch, in a tensionally stressed bar, is twice that of the average 
stress for the section, and that for sharper notches the stress 
might, at least for hard rubber, be as much as five times the aver- 



* Leon, ij'ber die Spannngsverteilung in einer halbkreisformigen Kerbe, Osterreichische Wochenschrift 
fiir den ciffentlichen Baudienst, 29, p. 43; 1908. 



LIWV^WY Ci^ CONGRESS 



Corrosion and Ductility of Brass 3 

age stress. The stress at the free edges of the notch is under such 
circumstances practically zero. The fiber stress along the rough- 
ened surface will therefore vary in value between zero, at the 
top of the small ridges, and a value, at the bottom of the furrows, 
much larger than that of the average stress. 

Now the electrolytic potential of a metal or alloy is increased* — 
i. e., made more electropositive — by the application of a stress. 
The results of some measurements and fiuther discussion of this 
effect of stress on the solution potential will be given below, under 
the next heading. 

It is therefore evident that after the formation of the furrowed 
or roughened surface on the brass specimen under tension, the 
emf will be greater at the bottom of the small nicks or notches 
than on the side of these notches immediately adjacent. Corro- 
sion will therefore, other things being equal, be more severe at the 
bottom of these notches than elsewhere, and the notches will 
grow inward, becoming sharper and sharper, quite in contrast to 
their behavior when uncorroded, since in a ductile material such 
notches tend to flatten or smooth out under tension, thus reducing 
the local fiber stresses at these points. 

In fact, Heyn^ finds that the behavior of a notch in a bar 
under tensional stress distinguishes brittle from ductile material. 
The notch in a ductile material tends to smooth out, whereas in 
the brittle material it becomes sharper and narrower under ten- 
sion, finally causing fractmre. Thus, the effect of corrosion under 
stress is to favor the growth and narrowing down of cracks and to 
cause an apparent brittleness of the brass. 

MEASUREMENT OF THE ELECTROLYTIC POTENTIAL OF BRASS UNDER 

STRESS 

The increase of electrol3rtic emf of a simple metal to a solution 
(containing ions of the same metal) due to the application of a 
stress may be calculated^ from the consideration that the in- 
crease of emf of a system is proportional to the increase of free 
energy, and that for isothermal, reversible processes the increase 
of free energy of a system is equal to the amount of work done on it. 
The application of stress of values below the true elastic limit is a 
reversible process and can be carried out isothermally. Consid- 

* C. Hambuechen, The Corrosion of Iron, Bull. Univ. Wisconsin, No. 8, 1900; T. Richards and Behr, The 
EMF of Iron Under Various Conditions, Publ. Carnegie Inst., No. 6r, 1906. 

6 E. Heyn. Materialienkunde, p. 376. 

6 Walker and Dill, The Effect of Stress on the EMF of Soft Iron, Trans. Amcr. Electrochem. Soc, 7, 
P- 153:1907- 



4 Technologic Papers of the Bureau of Standards 

ering, for the purpose of simple and approximate calculation, 
that brass is a simple metal of equivalent weight equal to 

63 + 65^ 
2x2 ^^' 

a stress of 25 000 pounds per square inch, applied isothermally 
to a brass rod, having a modulus of elasticity of 15 X 10 ® pounds 
per square inch, and a density of 8, should produce an increase 
of emf equal to 

work (in ergs) equivalent weight X lo"^ 
density 96 500 

2S 000 25 000 1000 X 980 

_o . ^ 2 X X 32 X 10-^ 

= 2 15 000 000 14.2 ^ 

96 500 X 8 
= 0.000006 volt; 

with greater stresses and after plastic yielding had commenced, 
an increase of 2 millivolts might be obtained, assuming that one- 
half of the energy of deformation is potentialized. 

Some measurements have been made of these quantities, with, 
however, contradictory results. Hambuechen (loc. cit.) finds for 
brass — composition of the brass and of the electrolyte not stated — 
an increase of 0.3 millivolt at the elastic limit, and an increase of 
about 4 millivolts at the point of rupture. For iron and steel he 
finds much larger values. Both Walker and Dill and Richards 
and Behr, on the other hand (loc. cit.), come to the conclusion 
that, although there is an increase in the emf of iron (they did not 
work with other metals) caused by application of stress up to and 
beyond the elastic limit, it is so small as to defy measurement; they 
explain Hambuechen's results on iron as having been due to the 
hydrolysis of the ferric salt, which he used as electrolyte. 

In the measurements made by the author use was made of two 
electrodes of the same material, and in the same condition; one of 
these could be stressed, the other served as a comparison electrode, 
measurement being made in each case of the difference of potential 
between the two. These were immersed side by side, in a solution 
contained in a large glass tube, with stopper at the bottom, the 
solution being contained between a layer of paraffin below (to 
which was sometimes added a slight amount of carbon tetra- 
chloride) and a layer of a transformer oil above; these precautions 
were taken to prevent oxygen from the air from affecting the emf. 



Corrosion and Ductility of Brass 



The arrangement is shown in the sketch, Fig. i; the potential 
measurements were made by a potentiometer. It was found 
necessary to protect the electrodes from light during measure- 
ments, by wrapping black cloth around the container. 

The material used in the tests was a homogeneous alpha brass 
of the following composition : 



Per cent 

Copper 67. 5 

Zinc 32. 5 

Tin 



Lead 

Iron 

Manganese. 



Per cent 

O. 06 

. o. 02 



stressed electrode ( a ) 



electrdlyte 



paraffine 



This material was most kindly fur- 
nished by the American Brass Co., in the 
form of one-fourth inch rods. Before 
the tests, this material was annealed at 
400° C for an hour in order to relieve any 
initial stress, and the surface thereupon 
prepared by light rubbing with fine emery 
paper, followed by etching with nitric 
acid, washing and drying. 

The test procedure consisted in placing 
the specimens in position, with the elec- 
trode (a) under zero stress, noting from 
time to time the emf between (a) and (b) , 
then applying a stress, in units of 10 000 
pounds per square inch, again noting 
during a certain period of time the emf, 
removing the stress, noting the emf, re- 
applying the stress, etc. The results of 
these measurements are best studied in 
the form of a curve, giving the emf as a 
function of the time, a change of stress 
being noted on the curve. Two such 
curves showing typical results obtained 
are given in the Figs. 2 and 3. 

It is seen that there is a small but 
unmistakable increase in the emf, due to 
the application of stress, and that this becomes relatively quite 
large, in Fig. 2, when the yield point of the brass is reached. 

It is now to be noted that when a bar of metal is subjected to a 
tensile stress a lowering of temperature takes place, and this 
lowering of temperature itself produces a momentary change in 




Fig. 



—Disposition of 
electrodes 



6 Technologic Papers of the Bureau of Standards 

emf, which often masks the permanent one; this effect can be 

noticed in the curves, and it is for this reason that the author 

adopted the method of holding the specimen at a constant stress 

for some time, and noting the emf, instead of using Hambuechen's 

method of continually increasing the stress during the course of 

the measurements. 

CONCLUSIONS 

It is believed that an increase in emf of alpha brass to solutions 



. i 












> 








1 


J 






^., 


















s 








i 




\ 


'. 




^^ 


> 






9 






















I 






i 

£ 






















" 






£ 


5 













Fig. 2. — The emf of stressed to unstressed brass in a solution of Njio CuSO^.ZnSO^. 
The + sigji indicates that the bar under stress is electropositive to the comparison 
electrode 

containing zinc and copper ions, caused by the application of 
tensile stress, has been indicated and measured. This amounts 
to about 0.2 millivolt for 20 000 pounds per square inch (below the 
elastic limit) and to about i millivolt at the yield point of the mate- 
rial, 30 000 pounds per square inch. This value is apparently 
much greater than would be calculated for elastic stresses from 



































1 
















f 


i 
I 






1 


. 






t 




1 






i 
s 


1 














i 


1 
S 


I 




i 


I 






J_ 










4^ 






! 








v^ 




' V 


' 




















u 






■"" 


































i 







Fig. 2>- — The emf of stressed to unstressed brass in a solution of N ZnSO^. The + indi- 
cates that the stressed bar is electropositive to the comparison electrode 

thermodynamic considerations; it agrees well with values calcu- 
lated for stresses above the elastic limit. The author does not 
wish at present to discuss this divergence otherwise than to point 
out that the emf measured may be that at some point of the 
surface, e. g., at the bottom of a small notch, where the fiber stress 
is much greater than the average; in that case these results could 
be brought into agreement with thermodynamic theory. 



Corrosion and Ductility of Brass 7 

An explanation is given of the effect of corrosion on brass under 
stress, in decreasing the ductility and strength, and which is based 
upon this fact of the increased emf and therefore corrodibility 
of brass under stress. At the bottom of small furrows in the rough- 
ened surface the stress is greater than at ridges immediately 
adjacent; a galvanic couple is formed, and the bottom of the 
furrow only is corroded, forming in time a crack, which becomes 
narrower and sharper as it penetrates inward, finally so reducing 
the cross section that fracture occurs. 

This conception of the cause of the embrittling effect of corroding 
agents on brass under stress is borne out by the examination of 
samples which have been subjected to Jonson's test. In such 
samples, not one but several fissures appear, narrowing down and 
becoming microscopically fine as they penetrate farther and farther 
into the interior of the specimen. It may be noted in this connec- 
tion that these fissures appear to favor the beta constituent and 
to avoid the alpha in a brass, such as manganese bronze, contain- 
ing both of these. 

The question as to why brass should be so susceptible to this 
effect, and not also iron and steel (apparently, at least) , can not at 
present be adequately answered, not indeed until further investi- 
gation has considerably increased our knowledge concerning the 
values of the stress-emf effect in these materials, the relative 
significance of this effect in comparison with other local emf's 
(due to oxide and slag inclusions, etc.) , the effect of the covering 
of corrosion product formed, and other points. 

Apparently the effect produced by corroding solution having a 
low conductivity will, other things being equal, be greater than that 
of one of high conductivity, since it allows greater prominence 
to those local stress emf's between portions of the material imme- 
diately adjacent. It is known, for example, that nitric acid does 
not produce cracking in initially overstressed brass in the manner in 
which ammonium hydroxide or mercurous nitrate or chloride 
does. 

Washington, July 12, 191 6. 



LIBRARY OP UUiMUMcoo 



019 423 382 



\ 



ADDITIONAL COPIES 

OF THIS PrBLICATlON MAY BE PROCUBED FROM 

THE SUPEECsTEXDENT OF DOCUMENTS 

GOVEENMENT PRINTIXG OFFICE 

■WASHINGTON, D, C. 

AT 

5 CENTS PER COPY 



A complete list of the Bureau's publications 
may be obtained free of charge on application to 
the Bureau of Standards, Washington, D. C. 



ill. 



